In the production of electronic devices, semiconductor die are assembled onto substrates to create a platform for interconnecting the die signal input and output functions to the other devices with which it will communicate. Typically, this assembly is effected with specifically designed solders and/or polymer adhesive disposed between the non-functional side of the die and the substrate.
In certain subsets of semiconductor die (typically used for power management or lighting applications), the sides of the die attached to the substrate are metallized to promote good thermal transfer between the operating semiconductor die and the substrate, and, in some cases, electrical interconnection as well. Silicon-based semiconductor die fabricated for power applications are typically metallized in this fashion in order to achieve both an electrical and thermal connection to the backside of the die. As device operating temperatures increase, additional materials, such as silicon carbide and gallium nitride, are being prepared in a similar fashion to improve device performance in elevated temperature environments
For applications in which the heat to be dissipated from the die is relatively low, metal-particle-filled polymeric adhesives are the most common class of materials used to adhere the semiconductor die to the packaging element. In these adhesive systems, thermal and, if desired, electrical conduction propagates through various mechanisms based on the volume fraction of metal filler. Research (and industrial practice) has shown that conductive adhesives with a volume fraction of metal filler of approximately 30% can achieve low electrical resistivity and high thermal conductivity. However, conductive adhesives have inherent limitations on their electrical reliability performance, and thus their ability to replace soft solder, in semiconductor devices. Since the thermal and electrical conduction efficiency is directly dependent on the proportion of filler particles relative to the polymer adhesive content, there is an upper limit in conductivity for these adhesives resulting from the practical limitations on the amount of filler that can be incorporated without compromising the mechanical integrity of the adhesive.
For applications in which higher power dissipation is required, the use of solders or silver-based sintering materials are more common for semiconductor attachment to packaging elements. Traditionally, lead-based solders are used for the die attachment of power semiconductors because these solders wet well to the metallized die and package element(s), provide excellent electrical and thermal conductivity and have high elongation to mitigate the differences in coefficient of thermal expansion (CTE) between the semiconductor die and the package element(s). In the last decade, there has been an initiative to replace the lead-based solders with lead-free solders. For specialty applications, gold-tin, tin-antimony, indium or bismuth-based alloys are used, but these are often expensive and frequently have mechanical or electrical performance limitations. For more broad-based power die attach applications, tin-based solders have been evaluated as a Pb-free solution. Typically, these tin-based solders are applied in a paste or wire-based form. During thermal processing, the individual particles of tin-alloy melt and fuse to one another and the metallized surfaces to form a monolithic joint. As the individual particles collapse and consolidate into a single molten mass, the volatile flux vehicle is excluded from the mass resulting in large centralized pockets of void space from which the flux volatiles have been evacuated. These pockets can create hot spots on the semiconductor die due to the lack of thermal conduction in that locality, or reduced electrical performance of the device due to reduced area of electrical contact. Although these void pockets are endemic to both leaded and lead-free solders, in the lead-free solders subsequent thermal excursions can cause the alloy to remelt which can result in further consolidation of the void pockets into very large vacancies. Further, the lead-free tin-based alloys do not exhibit the elongation characteristic of the lead-based solders and therefore do not effectively mitigate the CTE mismatch between the semiconductor die and the package elements.
One class of materials proposed to meet the requirements for power semiconductors are known as silver sintering pastes. Silver sintering pastes are comprised of silver particles, often in nanometer or micrometer sizes, in combination with a carrier. During thermal processing, the high surface energy of the nano-sized silver particles is leveraged to sinter the silver particles together at a temperature far below the melting temperature of silver metal. Organo-metallic additives may also be used in the carrier composition to promote sintering performance. Applied pressure, not a typical manufacturing protocol for semiconductor attachment, is often required during the thermal processing to achieve a sufficiently consolidated and mechanically robust joint. While this class of materials offers many desirable characteristics, these expensive materials typically do not bond well to many packaging element surfaces that are increasingly popular for cost reasons.
Accordingly, the industry is currently in search of an alternative class of materials for the attachment of metallized semiconductor die onto package elements. The desired attributes of the new material class is the use of existing deposition and processing infrastructure; high thermal conductivity (>20 W/mK); stable electrical resistance, robust adhesion, mechanical and electrical reliability through multiple thermal excursions including standard industry reliability testing; low void volume and small sized voids in the joint; differential coefficient of thermal expansion (C 1E) mechanical stress management; and low cost.